1. Field of the Invention
The present invention relates to a magnetic field sensor, in particular to the structure of an upper shield layer of a magnetic field sensor that utilizes magneto-resistive effect.
2. Description of the Related Art
Development of high density magnetic recording in hard disk drives has almost reached the 100 Gbpsi class on a mass production basis. In order to cope with the tendency toward high density magnetic recording, a magnetic field sensor that uses a magneto-resistive effect, such as a MR (Magneto-Resistive) element, has been used as a magnetic head. In particular, a GMR (Giant Magneto-Resistive) element using a spin valve (SV) film provides a highly sensitive magnetic head because of the large change in electric resistance to sense current that flows through the element in order to read magnetic records in a recording medium. A SV film is stacked films that include a ferromagnetic layer in which the direction of magnetization is fixed in one direction (this layer may also be referred to as a pinned layer hereinbelow), another ferromagnetic layer in which the direction of magnetization varies in accordance with an external magnetic field that is generated by a recording medium (this layer may also be referred to as a free layer), and a non-magnetic space layer disposed therebetween. In an SV film, the direction of magnetization of the free layer creates an angle relative to the direction of magnetization of the pinned layer in accordance with the external magnetic field, so that spin dependent scattering of conduction electrons varies depending on the relative angle, causing a change in magnetoresistance. The magnetic head detects the change in magnetoresistance to read magnetic information from the recording medium.
In an MR element, a free layer causes change in magnetoresistance in response to an external magnetic field, as described above, and in general, a bias magnetic field is applied to an MR element in order to achieve linear change in magnetoresistance and to reduce noise, and thereby to stabilize the output characteristics. In general, bias layers that apply a bias magnetic field are arranged on both sides of an MR element with respect to the track width direction so that the bias magnetic field is applied in the track width direction of the MR element. On the other hand, the external magnetic field is applied in a direction that is parallel to the layers and that is perpendicular to the track width direction. When no external magnetic field is applied, the magnetization of the free layer is oriented in the track width direction. If an external magnetic field is applied, the direction of magnetization turns in accordance with the magnitude of the external magnetic field.
An MR element is covered with shield layers on both sides with respect to the direction of stacking and is magnetically shielded from the surroundings in order to detect only the magnetic field that is generated by a predetermined recording domain on a recording medium. However, actually, the shield layers are magnetized by the surroundings, and as a result, the free layer is affected by the shield layers. The shield layers are magnetized by the external magnetic field that is applied by the recording medium, and are also magnetized due to deformation of the shield layer which is arranged adjacent to the recording head. The deformation is caused by the operation of the recording head. Specifically, heat which is generated through the writing operation of the recording head causes deformation (strain) in the upper shield layer, which in turn changes the magnetic domain structure in the upper shield layer by the inverse magnetostriction effect. The deformation in the upper shield layer that is caused by this effect may also be referred to as external deformation hereinbelow. Magnetization of the upper shield layer that is caused by these effects is unstable because it depends on the magnitude of the external magnetic field or the condition of heating in the coil. Therefore, even if a bias magnetic field is applied, the condition of magnetization of the upper shield layer may vary because of variation in the magnetic domain structure, especially when the latter effect dominates. As a result, the bias magnetic field is disturbed, resulting in difficulty in applying the desired bias magnetic field to the free layer and in achieving stable output characteristics.
In order to cope with the above problems, a technique has been disclosed in Japanese Patent Laid-open Publication No. 2000-48327, in which shield layers have multi-layer structures that are composed of magnetic layers and non-magnetic spacing layers interposed therebetween. Since the directions of the magnetization of the magnetic layers are in anti-parallel with each other via the non-magnetic spacing layers, the formation of a single magnetic domain is promoted, and the shield layer is less affected by the external magnetic field. Further, in order to minimize the disturbance to the magnetization of the upper shield layer that is caused by the external deformation, it is preferable that the magnetostrictive coefficient of the material be as small as possible. For example, Ni82Fe18, which has a magnetostrictive coefficient of 0, is used as the material for the upper shield layer.
However, the shield layer in a multi-layer structure which is described in the above patent document needs a complex fabrication process, and this causes an increase in cost. In a multi-layer structure, each layer must be formed such that the magnetic moment cancels each other out so that the layers, as a whole, do not leak magnetic field. However, in practice, it is difficult to perfectly prevent leakage of magnetic field, and therefore, it is also difficult to prevent influence on the free layer. In addition, in an actual element, it is difficult to fabricate an upper shield layer such that it has a magnetostrictive coefficient of 0, and therefore, the magnetostrictive coefficient varies depending on the locations on a wafer on which many MR elements are formed. Since the upper shield layer needs to be formed in a certain thickness to satisfy the functional requirement, even a small variation may result in a large magnitude of magnetization as a whole, causing a serious effect on the MR elements.